(a) Field of the Invention
The present invention relates to a needle journal bearing for the magnetically supported rotor shaft of a measuring instrument, such as a watt-hour meter, in which a thin metal bearing needle protrudes into a needle guide of a plastic material having good wear and friction properties.
(b) Description of the Prior Art
Such a needle bearig is described in German Auslegeschrift No. 12 11 054.
In magnetic support bearings, such needle journal bearings serve for the stable lateral guidance of the oppositely disposed pairs of magnet rings. It has been found that the bearing point between the bearing needle and the needle guide is subjected to increased wear which can be explained by the fact that vibration stress of the bearing occurs as well as stress due to rotation.
British Pat. No. 11 43 942 describes a cylindrical clapper and/or hollow cylindrical guide the outside surface of which is provided with PTFE coatings by applying this material in suspension to the surfaces and solidifying them at elevated temperature.
British Pat. No. 536 972 describes a sub-bearing for measuring instruments with a rotatable shaft in which the bearing bushing and the bearing pin are made of plastic and are connected to the shaft. The bearing pin is either fastened to the shaft at its end by means of a metal sleeve or can be pushed onto the shaft itself. The shaft is then supported and guided by the solid plastic bearing pin in this structure.
In the journal "Feinwektechnik", vol. 59.1955, no. 1, pages 10 to 15, the subject "Polyamides as structural materials in precision engineering" is discussed. Here, in connection with the rapid removal of friction heat, it is said that it is possible to melt a thin layer of polyamide onto a steel shaft. Preference is indicated for the use of the pair metal/polyamide as bearing materials. Melting polyamide onto metal with the high precision required for needle bearings is an expensive manufacturing process. In addition, the entire bearing element must be replaced if the polyamide coating on a bearing element is worn.
It is an object of the present invention to provide a needle journal bearing of the type mentioned above which makes it possible to support the rotor shaft with low friction and wear in a manner advantageous from a production point of view, while at the same time the accuracy of the guidance and good cooling of the bearing are to be preserved.